# How are the coefficients in the EGM96 model normalized?

For my own interest I'm trying to implement the EGM96 gravitational model. I found the coefficients here. The readme file says that they are "fully normalized", but I'm not 100% sure what this means.

For instance MATLAB defines fully normalized associated Legendre polynomials as: $$N_\ell^m = (-1)^m\sqrt{\frac{(\ell+0.5)(\ell-m)!}{(\ell+m)!}}P_\ell^m$$

whereas the Wikipedia article on spherical harmonics says that geodesy often uses the normalization: $$Y_\ell^m = \sqrt{\frac{(2\ell + 1)(\ell-m)}{(\ell +m)!}}P_\ell^m$$

The readme file uses the notation Ynm, which leads me to believe I should be using the second normalization. Is that correct?

• The correct answer is "none of the above." Both the MATLAB and wikipedia articles use complex spherical harmonics. The gravity model coefficients are real, separated into sine and cosine parts. The normalization is a bit different. Rather than worry about denormalization, it's best to use normalized coefficients throughout. I don't have time to write a good answer. (Continued). Nov 9 '15 at 0:13
• Nov 9 '15 at 0:13
• Caveat emptor: That paper is over 20 years old, and the Ada code (Ada? What's that?) in the paper has bugs. Except for locations directly over the poles, the equations in that paper are not buggy. Nov 9 '15 at 0:14
• @DavidHammen I'm not trying to denormalize the coefficients; but in order to use the normalized coefficients, I also have to use the normalized associated Legendre polynomials, correct?. In the paper you posted, the normalization they use is given in eq 3-11. Is this for sure the same one that's used in EGM96? Nov 9 '15 at 10:08
• Yes, this is the same one that's used in EGM96, and in every other spherical harmonics gravity model I've run across. See page 6 of csr.utexas.edu/grace/gravity/ggm02/GGM02_Notes.pdf . While models differ in how they handle tides, the coefficients have been standardized (de facto) for a long time. Nov 9 '15 at 11:47

The normalization factor conventionally used in satellite geodesy appears in [Kaula's Eq. 1.34]{https://store.doverpublications.com/0486414655.html}. Here I call it $$N_{n,m}$$: \begin{align*} N_{n,m} = \sqrt {(2-\delta_{0,m})(2n+1) \dfrac {(n-m)!} {(n+m)!}} = \begin{cases} \sqrt {(2n+1) \dfrac {(n-m)!} {(n+m)!}}, & \quad m = 0 \\ \sqrt {2 (2n+1) \dfrac {(n-m)!} {(n+m)!}}, & \quad m > 0 \end{cases} \end{align*}

TL;DR

Why normalize?

One compact formula for gravitational potential is \begin{align*} U & = \frac {G M}{r} \left[ 1 + \sum_{n=1}^{\infty} \left(\frac {a_e} {r}\right)^n \sum_{m=0}^{n} P_{n,m}(\cos\theta) \big[ C_{n,m} \cos(m \lambda) + S_{n,m} \sin (m \lambda) \big] \right] , \end{align*} where coordinates $$(r,\theta,\lambda)$$ are respectively the radial distance, colatitude, and longitude of the point where potential is to be evaluated, all expressed in a coordinate system fixed in the body and rotating with it. Product $$G M$$ is the body's gravitational parameter. Coefficients $$C_{n,m}$$, $$S_{n,m}$$ encode the body's gravitational field. The $$P_{n,m}(\cos\theta)$$ are Associated Legendre Functions (ALFs). Their argument $$\cos\theta$$ is the cosine of colatitude $$\theta$$ measured southward from the north pole, or equivalently the sine of latitude measured north and south from the equator. Degree $$n$$ and order $$m$$ are non-negative integers $$0, 1, 2, \dots$$ with $$0 \le m \le n.$$ Reference distance $$a_e$$ and body mass $$M$$ make $$C_{n,m}$$, $$S_{n,m}$$ dimensionless pure numbers.

For large $$n$$ or $$m$$, ALFs can become very large while coefficients $$C_{n,m}$$, $$S_{n,m}$$ become very small. To compensate, a normalization factor (denoted above as $$N_{n,m}$$) is introduced which becomes small with large $$n,m$$. The factor multiplies $$P_{n,m}(\cos\theta)$$ and divides $$C_{n,m}$$ and $$S_{n,m}$$ so that products $$C_{n,m} P_{n,m}(\cos \theta)$$ and $$S_{n,m} P_{n,m}(\cos \theta)$$ are unchanged within the potential formula. An overline above $$P$$, $$C$$, and $$S$$ distinguishes "fully normalized" quantities from their unnormalized counterparts: \begin{align*} C_{n,m} \, P_{n,m}(\cos\theta) & = (C_{n,m} / N_{n,m}) (N_{n,m} P_{n,m}(\cos\theta)) \equiv \overline{C}_{n,m} \, \overline{P}_{n,m}(\cos\theta) , \\ S_{n,m} \, P_{n,m}(\cos\theta) & = (S_{n,m} / N_{n,m}) (N_{n,m} P_{n,m}(\cos\theta)) \equiv \overline{S}_{n,m} \, \overline{P}_{n,m}(\cos\theta) . \end{align*} The only change to the potential formula is the overlines: \begin{align*} U & = \frac {G M}{r} \left[ 1 + \sum_{n=1}^{\infty} \left(\frac {a_e} {r}\right)^n \sum_{m=0}^{n} \overline{P}_{n,m}(\cos\theta) \big[ \overline{C}_{n,m} \cos(m \lambda) + \overline{S}_{n,m} \sin (m \lambda) \big] \right] . \end{align*}

• +1 This is one of those answers that's going to be really handy/helpful to have here!
– uhoh
May 29 '19 at 21:47